Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as nitrogen oxides (NOX), and solid particulate matter also known as soot. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOX and soot emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to ensure compliance with the regulation of NOX, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is a process where a gaseous or liquid reductant, most commonly urea, is injected into the exhaust gas stream of an engine and is absorbed onto a substrate. The reductant reacts with NOX in the exhaust gas to form H2O and N2. Although SCR can be effective, it is most effective when a concentration of NO to NO2 supplied to the reduction catalyst is about 1:1. In order to achieve this optimum ratio, a diesel oxidation catalyst (DOC) is often located upstream of the substrate to convert NO to NO2.
Another strategy used to reduce the emission of NOx is exhaust gas recirculation (EGR). EGR is a process where exhaust gas from the engine is recirculated back into the engine for subsequent combustion. The recirculated exhaust gas reduces the concentration of oxygen within the engine's combustion chambers, and simultaneously lowers the maximum combustion temperature. The reduced oxygen levels provide fewer opportunities for chemical reaction with the nitrogen present, and the lower temperature slows the chemical process that results in the formation of NOX. A cooler is commonly located within the EGR loop to cool the exhaust before it is received by the engine.
In order to ensure compliance with the regulation of soot, some engine manufacturers remove the soot from the exhaust flow using a particulate trap. A particulate trap is a filter designed to trap soot in, for example, a wire mesh or ceramic honeycomb media. One type of particulate trap utilized in conjunction with diesel engines is known as a diesel particulate filter (DPF). The soot accumulated within the DPF can be burned away through a process called regeneration. For this purpose a regeneration device, for example a fuel-fired burner, can be located upstream of the DPF.
When combining SCR, soot collection and EGR together into one system, special considerations must be taken into account. For example, if the exhaust gas recirculated back into the engine is taken from downstream of the DOC, the received exhaust may be relatively rich in NO2. As such, when the exhaust passes through the EGR cooler, some of the NO2 gas may mix with moisture that condenses within the cooler and form nitric acid that can be corrosive to components of the engine. In similar manner, if the EGR loop receives exhaust from downstream of a urea injection location, the condensing moisture within the cooler may mix with residual ammonia to form ammonium nitrate, which can be unstable when mixed with diesel fuel.
An exemplary system implementing the strategies described above is disclosed in U.S. Pat. No. 6,823,660 (the '660 patent) issued to Minami on Nov. 30, 2004. This system includes an oxidation catalyst located upstream of a DPF, which in turn is located upstream of an SCR catalyst. The system also includes an EGR passage to direct exhaust from an associated engine at a location upstream of the oxidation catalyst back into the engine.
Although effective at controlling the amount of NOX and soot exhausted to the environment, the previously described system may fail to account for all of the special considerations. That is, because the EGR passage of the '660 patent receives exhaust from upstream of the DPF, the exhaust directed back into the engine may contain large amounts of particulates that can mix with condensation in the cooler to form sulfuric acid. In addition, the particulates can be damaging to engine components.
The system of the present disclosure solves one or more of the problems set forth above.